1. Field of the Invention
The present invention relates to a controller that controls the die cushions of a press machine.
2. Description of the Related Art
A die cushion apparatus is used to alleviate changes in the pressure applied to a metal sheet, to avoid the sudden application of pressure when the sheet is clamped between metal dies by motion of the press axis of a press machine.
In conventional die cushion apparatus, hydraulic or pneumatic pressure is used to control the pressure applied to the metal sheet. The pressure is controlled by a controller of the die cushion apparatus after the upper die, which is mounted on the press axis of the press machine, is lowered and strikes the sheet metal. In die cushion control involving hydraulic or pneumatic pressure, however, there is a temporal delay from output of a control signal to response thereto, making it difficult to carry out control so as not to apply a surge of pressure due to an impact at the start of pressing to the metal sheet.
To solve this problem, Japanese Patent Application Laid-open No. 10-202327 proposes a die cushion apparatus having a servo motor that raises and lowers the die cushion, selectively carrying out position control of cushion stroke and pressure control by current torque control by using the servo motor.
In the above patent document, the touch point at which the upper die comes into contact with the cushion pads of the die cushion apparatus is recognized by detecting the change in the value of the current in the servo motor that drives the cushion pads. Accordingly, the switchover from position control to pressure control is delayed, causing the problem of inability to reduce the shock at the time of the touch on the cushion pads. During position control, the cushion pad driving servo motor holds the cushion pads at a preset position, and when the upper die strikes the cushion pads in this state, it tries to push the cushion pad down.
The servo motor, however, tries to hold the cushion pads at the commanded position, as a result, its driving current increases and therefore its output torque increases accordingly. Although switchover to pressure control is made upon detection of the increased driving current, the pressure has already increased, thereby affecting the metal sheet to be pressed.
In control of the switchover of the die cushion from position control, by which the die cushions are held in a prescribed position, to pressure control and vice versa, another problem is that at the return from pressure control to position control, the positional deviation that has accumulated during pressure control causes the servo motor to be driven suddenly, generating mechanical shock.